In wireless communication networks, mobile communication devices or UEs communicate in a point-to-point fashion with a base terminal station (BTS) by the exchange of radio signals. Because the BTS is coupled by a core back-haul network to other BTSes servicing other UEs as well as the internet, the UE is capable of voice and/or data communications with other UEs and of accessing the internet.
In 1G and 2G technologies, the BTS was typically sited at or near its associated antenna tower. The BTS generates an analog RF signal and propagates the RF signal by RF cable pair to the antenna.
Starting with 3G networks, a decoupled front-haul concept was introduced, which is shown in an example block diagram in FIG. 1, where a portion of the functionality for a plurality of BTSes is co-located at a remote distributed front-haul processor or baseband unit (BBU) 110. The centralization of some BTS functions in a BBU 110 or BBU pool allows economies of scale in some of the BTS processing and lowers transmission losses by the communication of baseband signals, between the BBU 110 and a wireless transceiver or remote radio head (RRH) 120 along a point-to-point optical fiber front-haul link 130 between them, which the RRH 120 converts to RF time-domain signals for transmission to the UE. The BBU 110 or BBU pool may be coupled to a mobile back-haul network 140 by one or more cables or fibers 145.
More recently, the decoupled front-haul concept has evolved into a cloud-RAN (C-RAN) architecture that takes advantage of developments in wireless, optical and IT technologies, including without limitation, common packet radio interface (CPRI) protocols across dense wavelength division multiplexing (DWDM) optical communications front-haul links 130, to interconnect the BBU 110 and the RRH 120. The C-RAN architecture also takes advantage of concepts drawn from software-defined networking (SDN) and network function virtualization (NFV). The C-RAN architecture permits low-complexity RRH 120 implementation, which is advantageous, especially for small cells.
FIG. 2 shows an example of processing of BTS functionality between the link layer or media access control (MAC) 210 of a core network and the antenna 220 of an RRH 120. Such functionality can be split between the BBU 110 and the RRH 120 in a number of fashions or splits 231-234, respectively designated (A) through (D).
Conventionally, the C-RAN concept contemplates a time domain I/Q split, shown as split (A) 231, in which the front-haul links 130 between the BBU 110 and the RRH 120 convey time domain signals. The time domain I/Q split 231 is fully centralized, so that the RRH 120 is only assigned a few functions. In some cases, the RRH 120 functions may include power amplification and RF mixing of signals between baseband and RF. In some cases, the BBU 110 may digitize signals so that the front-haul links 130 convey digital signals such as point-to-point continuous bit-rate (CBR) signals using CPRI over an Ethernet or radio over Ethernet (ROE). In such cases, the RRH 120 functions may also include digital-to-analog conversion (DAC) functions on the transmit side and analog-to-digital conversion (ADC) functions on the receive side.
Such CBR signals consume considerable front-haul capacity, on the order of 1 Gbps per 20 MHz channel, irrespective of the UE load borne by the RRH 120 in the uplink direction and by the BBU 110 in the downlink direction. That is, the antenna 220 of the RRH 120 is always transmitting, even if the RRH 120 has a low density of UEs that it is supporting, that is, a low UE load.
Moreover, the front-haul capacity scales substantially linearly with a number of factors, including without limitation, the number of antenna ports, the number of sectors, the sampling rate, the number of carriers, the front-haul overhead, the front-haul compression factor and the I/Q bit-width. Furthermore, the CBR signals are constrained by latency factors, including a 3 ms hybrid automatic repeat request (HARQ) deadline for each sub-frame.
If the split in the C-RAN architecture is adjusted from a time domain I/Q split (A) 231 to a frequency domain I/Q split (B) 232, the front-haul links 130 can convey digital frequency-domain samples between the BBU 110 and the RRH 120. Such a modification is less centralized in that the RRH 120 is assigned additional functionality to locally handle all broadcast channel data and reference signals. Such functionality may include, without limitation, performing inverse fast Fourier transformation (IFFT) and mapping to convert frequency-domain samples to time-domain signals on the transmit side and fast Fourier transformation (FFT) and de-mapping to convert time-domain signals to frequency-domain samples on the receive side. Such an approach may impose a minor increase in front-haul overhead, since the front-haul would maintain a map identifying which I/Q sample is associated with a given sub-carrier of the analog RF signal.
The slight decrease in centralization may be justified since adjusting the split in this manner reduces the bandwidth of the signals along the front-haul links 130 considerably, on the order of as much as 50%. By way of non-limiting example, for each 20 MHz channel, there is about a 42% saving related to decreased number of I/Q samples in the frequency domain. Additionally, if cyclic prefix (CP) I/Q samples are not sent along the front-haul link, but added at the RRH 120, an additional 7% saving may be realized. Furthermore, such an adjustment in the split does not substantially affect the latency constraints relative to the conventional time domain I/Q split (A) 231. Still further, the physical broadcast channel (PBCH) and reference symbols can be omitted.
Additionally, it has been recognized that if a particular sub-carrier is not being used, in a frequency domain I/Q split (B) 232, the frequency sample(s) associated with the sub-carrier can be compressed or even discarded, with the result that the bandwidth across a front-haul link 130 between the BBU 110 and an RRH 120 may vary to some extent with the UE load borne by the RRH 120 in the uplink direction and by the BBU 110 in the downlink direction.
Other less-centralized splits include split (C) 233, in which forward error correction (FEC) and multiple-in multiple-out (MIMO) antenna processing (joint decoding, not joint detection) is maintained in the BBU 110, while the RRH 120 is assigned functions related to (de)interleaving, (de)modulation and equalization. In this split, front-haul capacity is dependent upon factors including without limitation, the signal plus interference to noise ratio (SINR), channel rank due to HARQ, the link adaptation algorithm employed and the UE load.
Finally, split (D) 234 assigns all of the BTS functionality at the RRH 120, removing all centralization and substantially reverting to the 1G/2G decentralized concept where the BTS is housed at the antenna site. In this split, front-haul capacity is tied to the user throughput and is dependent on factors including without limitation, the radio link quality and the UE load.
For purposes of explanation and not limitation, specific details are set forth in order to provide a thorough understanding. In some instances, detailed descriptions of well-known devices, circuits and methods are omitted so as not to obscure the description with unnecessary detail.